Multiple myeloma (MM) is an incurable cancer of plasma cells that is dependent on the bone marrow microenvironment for progression. Transforming growth factor-beta (TGF-) is a multi-functional growth factor elaborated by MM cells and by cells in the bone marrow microenvironment. TGF- stimulates MM progression through promotion of catabolic bone remodeling, IL-6 secretion, and Th17 T cell development, leading to osteolytic bone disease and immune dysregulation. Despite the importance of TGF- in MM, there are no clinical trials of TGF- antagonists for the treatment of MM. Most TGF- antagonists broadly target the ligand, receptors, or downstream kinases, which can antagonize homeostatic levels of TGF- and increase the risk of adverse events. We have developed a novel approach to selectively targeting disease-related TGF- activity in the MM bone marrow microenvironment through targeting only the TGF- which is activated through binding to the extracellular matrix protein, thrombospondin1 (TSP1). TSP1 is a secreted and extracellular matrix protein, which controls TGF- activity in disease by binding and activating latent TGF-. TSP1 is increased in MM. Our studies show that TSP1 activates latent TGF- expressed by human and mouse MM cells in vitro and importantly, a tetrapeptide antagonist (LSKL) of TSP1-dependent TGF- activation reduces MM tumor burden, stromal IL-6, and osteolytic bone disease in mouse models of MM. LSKL nearly completely blocks active TGF- in bone marrow cells of treated MM mice, indicating that the TSP1-TGF- antagonist peptide is working through targeting TSP1-dependent TGF- activation in the bone marrow microenvironment. These data establish that TSP1 is an important regulator of TGF- activity in MM and suggest that blockade of this pathway represents a novel and selective therapeutic strategy to reduce MM progression and bone disease. Our goal is to develop an orally available, druggable form of the LSKL peptide for treatment of MM. We have identified a lead compound (SRI31277) based on LSKL which has dose-dependent in vivo activity in a mouse model of MM, improved pharmacokinetics and oral bioavailability, and which will be the basis for identification and optimization of lead drugs. This proposal will combine mechanistic studies (Aim 1) with drug development efforts (Aim 2) to achieve our goal of identifying an orally active lead compound for treatment of MM. In Aim 1, we will further determine the role of the TSP1-TGF- pathway in MM through use of immune competent and TSP1 null models, by comparison of SRI31277 to global TGF- inhibitors and by use in drug combinations, and by complementary in vitro studies to define the TSP1 receptor on MM cells for TGF- activation and the role of this pathway in MM cell cytokine production and osteoblast/osteoclast regulation. The key goal of Aim 2 is to identify an orally active derivative of SRI31277 and a back-up peptide mimetic/small molecule suitable for GLP-IND enabling studies using both peptide and peptidomimetic/small molecule approaches.